The application generally relates to positive-displacement compressors. The application relates more specifically to controlling the volume ratio of a screw compressor.
In a rotary screw compressor, intake and compression can be accomplished by two tightly-meshing, rotating, helically lobed rotors that alternately draw gas into the threads and compress the gas to a higher pressure. The screw compressor is a positive displacement device with intake and compression cycles similar to a piston/reciprocating compressor. The rotors of the screw compressor can be housed within tightly fitting bores that have built in geometric features that define the inlet and discharge volumes of the compressor to provide for a built in volume ratio of the compressor. The volume ratio of the compressor should be matched to the corresponding pressure conditions of the system in which the compressor is incorporated, thereby avoiding over or under compression, and the resulting lost work. In a closed loop refrigeration or air conditioning system, the volume ratio of the system is established in the hot and cold side heat exchangers.
Fixed volume ratio compressors can be used to avoid the cost and complication of variable volume ratio machines. A screw compressor having fixed inlet and discharge ports built into the housings can be optimized for a specific set of suction and discharge conditions/pressures. However, the system in which the compressor is connected rarely operates at exactly the same conditions hour to hour, especially in an air conditioning application. Nighttime, daytime, and seasonal temperatures can affect the volume ratio of the system and the efficiency with which the compressor operates. In a system where the load varies, the amount of heat being rejected in the condenser fluctuates causing the high side pressure to rise or fall, resulting in a volume ratio for the compressor that deviates from the compressor's optimum volume ratio.
Volume ratio or volume index (Vi) is the ratio of volume inside the compressor when the suction port closes to the volume inside the compressor just as the discharge port opens. Screw compressors, scroll compressors, and similar machines can have a fixed volume ratio based on the geometry of the compressor.
For best efficiency, the pressure inside the chamber of the compressor should be essentially equal to the pressure in the discharge line from the compressor. If the inside pressure exceeds the discharge pressure, there is overcompression of the gas, which creates a system loss. If the interior or inside pressure is too low, back flow occurs when the discharge port opens, which creates another type of system loss.
For example, a vapor compression system such as a refrigeration system can include a compressor, condenser, expansion device, and evaporator. The efficiency of the compressor is related to the saturated conditions within the evaporator and the condenser. The pressure in the condenser and the evaporator can be used to establish the pressure ratio of the system external to the compressor. For the current example, the pressure ratio/compression ratio can be established to be 4. The volume ratio or Vi is linked to the compression ratio by the relation Vi raised to the power of 1/k; k being the ratio of specific heat of the gas or refrigerant being compressed. Using the previous relation, the volume ratio to be built into the compressor geometry for the current example would be 3.23 for optimum performance at full load conditions. However, during part load, low ambient conditions, or at nighttime, the saturated condition of the condenser in the refrigeration system decreases while the evaporator condition remains relatively constant. To maintain optimum performance of the compressor at part load or low ambient conditions, the Vi for the compressor should be lowered to 2.5.
Therefore, what is needed is a system to vary the volume ratio of the compressor at part load or low ambient conditions without using costly and complicated devices such as slide valves.